General Report

Semiconductor Spin Electronics
By Hideo OHNO*, Fumihiro MATSUKURA and Yuzo OHNOThis paper reviews the basic principles of
how spin can be employed in semiconductor
electronics, and reports on a number of possible applications of semiconductor spin electronics where progress is currently being made.
First we will present an overview of the spin
phenomena that occur in III-V ferromagnetic
semiconductors and their heterostructures (thestarting point for the new development), andin non-magnetic semiconductor quantum
wells and nanostructures. We will then discuss optical spin devices such as optical isolators, and electron spin devices such as magnetoresistance elements. Finally, we will review
proposals for quantum spin devices that utilize the quantum mechanical nature of spins.
Keywords: semiconductor, spin, ferromagnetic semiconductor, III-V
compound semiconductor

1. Introduction
The remarkable developments currently
being made in the fields of electronics and information technologies have been made possible by exploiting the properties of electron
charge and spin. Integrated circuits used fordata processing use the charge of electrons insemiconductors, while data storage media
such as hard disks use the spin of electrons ina magnetic material. Recently, various typesof spin polarization have been formed in semiconductors, such as carrier spin, spin in introduced magnetic atoms, and nuclear spin of
constituent atoms, and it has been shown thatnew functions can be implemented by injecting, transporting and controlling these spinstates. This is resulting in the emergence of a
new field-semiconductor spin electronics (orsemiconductor spintronics)-involving the useof spin states inside semiconductor materials.
The semiconductors currently used in integrated circuits, transistors and lasers, such as
silicon and gallium arsenide (GaAs), are non-magnetic in which the carrier energy is almost
independent of the spin direction. However, innanostructures where devices approach the limits of miniaturization, exchange interactions
have more pronounced effects and the presence of spin becomes more tangible. In magnetic semiconductors, which have both properties of magnetic materials and semiconductors, these exchange interactions can give riseto pronounced spin-related phenomena-not
just in nanostructures but also in more conventionally sized devices. Advances in materials science has led to the development of ferromagnetic semiconductors based on the III-V
compound semiconductors already used forelectronic devices,1,2) and due to progress madein the research of non-magnetic semiconductor quantum structures and nanostructures, it
is becoming possible to clearly express and control the degrees of freedom in charge and spin.
Semiconductor spin electronics may be divided into two fields. One might be referred
to as semiconductor magneto-electronics,
where attempts are being made to implement
new functions by incorporating magnetic
functions into semiconductors by using magnetic semiconductors (semiconductor materials that are also magnetic) or combinations ofsemiconductors and magnetic materials. For
example, it is thought that this field will leadto the realization of semiconductors devices
such as optical isolators, magnetic sensors andnon-volatile memories that can be integrated
into semiconductor devices and circuitry. If themagnetism or spin can be controlled by lightor electric fields, then it should be possible todevelop entirely new devices having functions
that have not been available before.
The other field is probably best referred
to as semiconductor quantum spin electronics. In this field, the main focus is on using thequantum mechanical nature of spin in semiconductors. For example, since the various
types of spins in non-magnetic semiconduc-tors have a much longer coherence time thanelectrical polarization and can be controlled
by light or electric fields, it is comparatively
easy to manipulate spin as a quantum-mechanical entity. Such properties lend themselves to the development of solid-state quantum information processing devices. In thisway, spin in semiconductors is heralding a new
era both in classical and quantum physics andtechnology.
Section 2 of this paper discusses the new
ferromagnetic semiconductors, Section 3 discusses the physical properties of spin in semiconductor structures, and sections 4, 5 and 6
discuss the applications of optical, electronicand quantum devices respectively. Our conclusions appear in section 7.

2. New Ferromagnetic Semiconductors
In this section we will present an overview
of the ferromagnetic properties observed indiluted magnetic semiconductor (DMS) materials, where a portion of the atoms in a nonmagnetic semiconductor are substituted bymagnetic atoms (thus they are called diluted
magnetic semiconductors). Especially, we will
describe the properties of III-V DMS materials.
III-V DMS’s exhibit ferromagnetism andprovide new degrees of freedom based on themagnetic cooperative phenomena in the semiconductor heterostructures used in transistors
and lasers. By using an epitaxially-grown III-V
magnetic/non-magnetic heterostructure, it becomes possible to look for new phenomena
such as spin-dependent tunneling, magnetoresistance, and spin-dependent light emission.
In this sense, it can be regarded as a prototype for semiconductor spin electronics, fromwhich we can learn what can be done by thespin degree of freedom in semiconductors.

* Laboratory for Electronic Intelligent Systems, Research Institute of Electrical Communications, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577.
E-mail: ohno@riec.tohoku.ac.jp. Classification Nos. 3.2, 3.7

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Adobe PageMaker 6.5J/PPCThe easy axis of magnetization in (Ga,Mn)
As grown on GaAs (001) planes is oriented
within the plane due to lattice strain.10) The
lattice constant of (Ga,Ma)As is slightly larger
than that of GaAs resulting in compressive
strain. Using a buffer layer of (In,Ga)As, which
has a larger lattice constant than (Ga,Mn)As
grown on it, the sign of the lattice strain can
be changed to allow the easy axis of magnetization to be oriented perpendicular to theplane.16)
The ferromagnetic transition temperature
can be determined in various ways, such as:(1) the temperature dependence of remanent
magnetization, (2) an Arrott plot (M2-B/M plot)using the magnetic field dependence of themagnetization near the transition temperature, or (3) a Curie-Weiss plot of the magnetic susceptibility in high-temperature paramagnetic regions. These methods all produce
almost the same TC values.17) The relationship

0.03

0.5

2.2.2 Magnetotransport characteristics
Depending on the Mn composition,(Ga,Mn)As exhibits either metallic conduction
or insulator-type conduction.11,19) Figures 1(a)and (b) show the magnetotransport characteristics (Hall resistance RHall and sheet resistance Rsheet) of a sample with metallic conduction (x=0.053, sample thickness = 200 nm).
As Fig. 1(a) shows, the anomalous Hall effectwhich is proportional to the magnetization
perpendicular to the plane-dominates in(Ga,Mn)As. Since the magnetization is small
in a DMS thin film, magnetization measurement using the anomalous Hall effect is better in terms of measurement sensitivity thandirect magnetization measurement. The mag-x = 0.053 2 K
B ⊥ plane 55 K

100 K
125 K

25 K

300 K
0.00

100 K
125 K
55 K

0.4
2K

25 K
300 K

0.3
-5

0

5

Magnetic field B (T)
0.08

(c)
0.06
0.04
0.02
0.00
0

100

200

1/χ Hall (arb. units)

The equilibrium solubility of magnetic impurities in III-V semiconductors is low, and under ordinary crystal growth conditions it is impossible to introduce a high density of magnetic atoms. In 1989, Munekata et al. used
low-temperature molecular beam epitaxy (LTMBE; deposition temperature approx. 250°C)
to achieve non-equilibrium crystal growth, andthereby succeeded in suppressing the surface
segregation of Mn and the formation of second phases, allowing them to perform epitaxial growth of an alloy of InAs and Mn,(In,Mn)As on a GaAs substrate.7) Subsequently,
ferromagnetic properties were discovered in
p-type (In,Mn)As,8) and in 1996 the growth of(Ga,Mn)As9)-a GaAs-based DMS-and ferromagnetic transitions in p-type (Ga,Mn)As werereported.10) The highest ferromagnetic transition temperature Tc of (Ga,Mn)As obtained so
far is 110 K (Mn cocentration x=0.053).11)
Since (Ga,Mn)As can be epitaxially grown
on a GaAs substrate, it is compatible with
GaAs/(Al,Ga)As quantum structures. It has
therefore become an indispensable material
for the study of semiconductor spin electron-

between TC and x is given by TC = 2000x K up
to around x = 0.053, and further increase of x
decreases TC.11) The reason for this decrease is
not understood. It has been confirmed that
(Ga,Mn)As with its easy axis of magnetization
perpendicular to the plane has a striped magnetic domain structure like an ordinary ferromagnetic material.18)

2.2.1 Magnetic properties

Hall resistance, R Hall (k Ω)

2.2 III-V ferromagnetic semiconductors

ics, and is being actively researched in recent
years.12-15)

Sheet resistance, R sheet (kΩ)

In the 1980s, research in DMS focused
mainly on II-VI semiconductors such as(Cd,Mn)Te and (Zn,Mn)Se.3) In a II-VI semiconductor, the II element is substituted byan equivalent valence of magnetic transition
metal atoms, making it possible to achievea high density of magnetic atoms and to fabricate quantum structures. In a II-VI DMS,
the optical characteristics such as Faraday
effect are greatly modulated by magnetic
fields due to the exchange interaction (sp-dinteraction) between the s and p orbitals of
nonmagnetic atoms and the d orbitals of themagnetic atoms. 3) However, it is generally
difficult to control conduction by doping in
II-VI semiconductors, which is one of the
major obstacles for the use of II-VI semiconductors as electronic materials.
Although II-VI DMS generally exhibit antiferromagnetism, spin-glass and/or paramagnetism properties, as we will discuss later, it
was recently reported that a p-doped II-VI DMS
undergoes a ferromagnetic transition at low
temperatures.4,5) In IV-VI DMS (Pb,Sn,Mn)Se,
ferromagnetism was also observed at low temperatures, which is attributed to the holemediated RKKY interactions.6)[RHall/Rsheets ]s

2.1 Diluted magnetic semiconductors

300

Temperature T (K)
Fig. 1 Magnetotransport characteristics in (Ga,Mn)As (Mn composition x = 0.053, 200 nm thick).
(a) Temperature dependence of the Hall resistance RHall. The magnetic field B is applied perpendicular to the surface ofthe sample. The anomalous Hall effect, which is proportional to the magnetization perpendicular to the plane, ispredominant. (b) Temperature dependence of sheet resistance Rsheet. The resistance value increases due to critical scatteringclose to the ferromagnetic transition temperature of 110 K. The negative magnetoresistance effect also reaches its maximum
near the transition temperature. (c) The temperature dependence of spontaneous magnetization as determined by an
Arrott plot of RHall/Rsheet, which is proportional to magnetization (black circles), and a Curie-Weiss plot (white circles). Here,χHall is the magnetization ratio determined from the anomalous Hall effect. The solid lines show the results of curve fitting
to the temperature dependence of spontaneous magnetization calculated from the Brillouin functions. This figure showsthat the ferromagnetic transition temperature is about 110 K.

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netization determined by the anomalous Hall
effect may be weighted by the conductivity,
however.
Using the anomalous Hall effect term, it
is possible to determine TC from the Arrott plot
(T ~ Tc) and Curie-Weiss plot (T >>Tc). Figure1(c) shows the temperature dependence of thequantity (RHall/Rsheet)s which is proportional to
the spontaneous magnetization determined by
the former method, from which it can be seenthat TC is 110 K. The value of TC determinedfrom the transport characteristics agrees wellwith the value obtained from magnetization
measurements.

well with the experimental value. Ferromagnetism is difficult to occur in n-type materials,
because of the smaller magnitude of the s-dinteraction N0α – 0.2 eV (where α is the s-d
exchange integral) of DMS. The fact that theeasy axis of magnetization of (Ga,Mn)As isstrain-dependent and the peculiar temperature dependence of magnetic circular dichroism24) can also be explained in terms of thevalence band structure using this model. On
the other hand, it has been pointed out that
other factors such as low energy spin-wave
excitation and disorder must be consideredwhen calculating Tc.26)

2.3.2 Models based on d-electrons
2.3 Mechanisms of ferromagnetism
2.3.1 A model based on the mean field
theoryIn a model based on the mean field theory,
the temperature dependence of the magnetic
susceptibility above the transition temperature
is assumed to be given by the Curie-Weiss law,and the sp-d interactions are regarded as theeffective magnetic field acting on the carriersystem. When spontaneous magnetizationand holes are present, spin-splitting occurs in
the valence band and the energy of the carrier system decreases as a result. At the sametime, the spontaneous magnetization increases the free energy of the localized magnetic spins. This free energy penalty decreases
with lowering temperature and at a certain
temperature the energy gain and loss balances. This is TC by the mean field model.20)
This is known as Zener ferromagnetism.21) It
should be noted that this is not the samemechanism as the double exchange interaction also proposed by Zener.
We used a 6×6 Luttinger-Kohn hamiltonianthat takes p-d interactions into account to calculate the carrier energy in the valence band.
Only the valence band is taken into accountas the conduction of (Ga,Mn)As is p-type. In
the calculation for (Ga,Mn)As we assumed theband parameters to be those of GaAs, andwe used a value of N0β = -1.2 eV for the p-dinteraction determined from core-level photoemission spectroscopy (N0 is the density of
cation sites, and β is the p-d exchange integral). 22) Also, in the calculation, we used anenhancement factor of 1.2 due to the carriercarrier interaction calculated from the local
spin density approximation.23) The value of TC
calculated in this way agrees well with the experimental value. The value of TC for (Zn,Mn)Te
calculated in the same framework also agrees

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By first principle calculation of the electronic states of (In,Mn)As, Akai showed thatwhen holes are present, the half-metallic ferromagnetic state becomes stable. He discussed
the ferromagnetism being as a result of double
exchange interactions caused by hopping ofd holes.27)
First principle calculation also has shownthat the ferromagnetic state is stable in a hypothetical zincblende GaAs/MnAs superlattice.
Again, the half-metallic state is predicted.28, 29)
The double resonance mechanism is also proposed for the ferromagnetism of (Ga,Mn)As,
from the electronic state calculated by the tightbiding approximation.30)

2.4 Room-temperature ferromagnetic semiconductors
Here we discuss the factors that should
be considered in order to achieve TC higher
than room temperature. According to themean field model, it is first essential to increase
the density of magnetic atoms x and the holedensity p (and it is also necessary to satisfyx>p).25) For example, in (Ga,Mn)As, if a holedensity of p=3.5×1020 cm-3, which is achievedin the x=0.053 sample, can be obtained, thenit should be possible to achieve a Tc beyond
room temperature by increasing the Mn density to x=0.15. Assuming it is possible toachieve a magnetic atom density and holedensity similar to those of (Ga,Mn)As, then ahigh Tc is expected in wide-gap semiconductors such as GaN or ZnO. This is because of
the light mass of the constituent elements andthe small lattice constant, which result in asmall spin-orbit interaction, a large effective
carrier mass, and a large p-d exchange interaction (N0β ∝ 1/a3, where a is the lattice constant).25)
First principle calculations have shown thatferromagnetism is stable in a DMS based on awide-gap semiconductor. When ZnO is dopedwith a high concentration of a transition metal,
doping with Mn results in antiferromagnetism(or ferromagnetism if simultaneously dopedwith holes), while V, Cr, Fe, Co and Ni result in
half-metallic ferromagnetism, and Ti and Cu
result in paramagnetism.31) The first-principle
calculation also shows that the ferromagnetic
state is stable for V, Cr, or Mn doped GaN.32)
A modulation-doped superlattice structure
has been proposed in which a well is formed
by a II-V DMS (Cd,Mn)Te, in which the hostatoms can be replaced by a high density ofmagnetic atoms, and a barrier is formed by
the C-doped III-V semiconductor AlAs. It has
been shown by first principle calculation that
holes supplied from carbon acceptors accumulate in the (Cd,Mn)Te well, making the ferromagnetism stable.33)

2.5 II-VI and other ferromagnetic
semiconductors
Advances in techniques for doping in
II-VI semiconductors have made it possible
to achieve carrier densities in excess of 1019
cm -3. 34) Shortly after a theory of ferromagnetic transition based on p-d exchange interactions was put forward, 35) ferromagnetism was observed below 1.8 K in modulation doped (Cd 0.975 Mn 0.025 )Te quantum
well (8 nm, p=2 × 10 11 cm -2 ). 4) It has also
been observed that a ferromagnetic transition takes place in a thin film (about 500
nm thick) of (Zn 1-x Mn x )Te and (Be 1-xMn x )Te(where x≤0.1) with p = 1019 -1020 cm -3 (T c <3 K).5,36)
A succession of recent reports have mentioned ferromagnetic transition temperatures
above room temperature-320 K in a II-IV-V2
chalcopyrite (Cd1-xMnx)GeP2,37) 290-380 K in
(Zn,Co)O, 38) room-temperature ferromagnetism in TiO 2:Co,39) and over 400 K TC’s in
CrAs and CrSb with a zinc-blende structure.40)
There is little doubt that further advances will
be made in the future.

3. Spin Properties in Non-Magnetic Semiconductor Structures
Along with the developments in ferromagnetic semiconductors, great progress in understanding and manipulating spin properties
in non-magnetic semiconductor structures
have also been made over the last decade. Thissection describes important aspects of spindependent phenomena in non-magnetic semi-

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3.2 Injection of spin into semiconductors
Spin injection is a critical requirement forthe development of spintronics.50-52) Schmidt
et al. analyzed the spin injection in a ferromagnetic metal/two-dimensional electron(semiconductor)/ferromagnetic metal structure
in a diffusive regime, and pointed out thatsince the semiconductor parts-where theelectrical conduction is not dependent on
spin-have high resistance, the overall change
in resistance is very small when the magneti-

3.3 Spin-dependent transportcharacteristics in quantum dots
In quantum dots where the electron transport is dominated by the Coulomb blockade,it is possible to control the number of occupying electrons (even or odd) by varying the gate
voltage.57) In such few electron systems, spin(magnetism) plays a predominant role in thetransport properties via exchange interaction.
This behavior is expected to prove useful for
functions such as spin filters and spin memories.58,59) Recently, exchange interactions between a quantum dot with a controlled spinstate and an electrode (the Kondo effect) havealso been observed.60,61)

3.4 Spin coherence
When a magnetic field is applied perpendicular to the carrier spin, the spin undergoes
Larmor precession about the field. The dynam-(a)

H

GaMnAs (p)+hGaAs spacer (i)dσ+InGaAs (i)GaAs (i)GaAs buffer (n)GaAs substrate (n)

(b)
1

0

E = 1.34 eV, d = 140 nm
I = 2.8 mA
Sweep up/down
T=6K
T=16K
T=31K
T=52K
8
1.0
6
4

0.5

2

-1
0.0
0

20

40

60

M (arb. unit)

Carrier spin relaxation in semiconductors
has been studied both theoretically and experimentally,41) and in recent years the spin relaxation times have been extensively studied
by time-resolved optical measurements using
femtosecond pulse lasers. Major spin relaxation
mechanisms in semiconductors include spinorbit interactions originating from the lack of
the inversion symmetry (the D’yakonov-Perel’effect), band-mixing (the Elliott-Yafet effect),and electron-hole exchange interaction (theBir-Aronov-Pikus effect).41) The relative importance among these mechanisms depends not
only on such material properties as the spinorbit coupling and the fundamental band gap,
but also on parameters as dimension, temperature, kinetic energy, scattering time, anddoping.
In a quantum structure, the degeneracyof light and heavy holes is lifted, and it is thus
possible to achieve 100% spin-polarized carriers by resonant excitation of electron-heavy
hole exciton with circularly polarized light. The
introduction of quantum confinement also
modifies such factors as mobility,42) symmetry,43-45) excitonic effects,46) locality,47) and doping48), resulting in a significant change of thespin relaxation process. In GaAs, for example,
the reported electron spin relaxation time are
distributed over a wide range from a few ps
to several tens of ns. 49) In particular, theD’yakonov-Perel’ effect (which predominatesat room temperature) depends strongly on the
crystallographic orientation of the growth direction.45)(Ga,Mn)As and n-GaAs, as shown in Fig. 2(a).
As Fig. 2(b) shows, we observed hysteresis in
the degree of polarization of the emitted lightat temperatures below the Tc of (Ga,Mn)As,and confirmed that spin injection takes place
in a zero field.55)∆P (%)

3.1 Spin relaxation in quantum
structureszation of the ferromagnets is changed fromparallel to anti-parallel. According to thismodel, it is difficult to achieve efficient spin
injection into semiconductors in a diffusive
regime unless the degree of spin polarizationin the ferromagnetic metal is close to 100%.53)(Rashba pointed out that the use of tunnel
junction for spin injection can be used to overcome the difficulty associated with the diffusive transport.53))
On the other hand, since magnetic semiconductors can be grown epitaxially on semiconductors and their conductivity is of a similar order to that of non-magnetic semiconductors, it is expected that highly efficient spin
injection and a large magnetoresistance canbe obtained when they are used as spinpolarizers and spin analyzers. The injection ofspin in p-n junctions using magnetic semiconductors has recently been confirmed by measuring the polarization of the light emitted asa result of the recombination of the injectedspin-polarized electrons (holes) and unpolarized holes (electrons).54-56) We have fabricateda light-emitting p-n diode consisting of an(In,Ga)As quantum well sandwiched between
a p-type ferromagnetic semiconductor

Relative Change in Polarization ∆P (%)

conductors that are essential for utilizing electronic and nuclear spin degrees of freedom,which involve spin relaxation and spin injection in quantum structures, spin-dependenttransport, and spin coherence.

0
80 100

T (K)

-0.01

0.00

0.01

Magnetic Field (T)Fig. 2 Injection of spin-polarized holes into a light-emitting p-n diode using a ferromagnetic semiconductor (Ga,Mn)As.
(a) Sample structure. Spin-polarized holes h+ travel through the non-magnetic GaAs and recombine with spin-unpolarized
electrons in the (In,Ga)As quantum well. I represents the current, and σ + represents circularly polarized light emitted
from the edge of the quantum well. (b) The dependence of the polarization ∆P of the emitted light on the magnetic
field B at each temperature. The solid and hollow symbols represent the degree of polarization when the magnetic fieldis swept in the positive and negative directions, respectively. The magnetic field was applied parallel with the easy axis ofmagnetization of the (Ga,Mn)As. The inset depicts the temperature dependence of the residual magnetization M in
(Ga,Mn)As, where the degree of polarization of the zero magnetic field seen in the emitted light (squares) exhibits thesame temperature dependence as the magnetization.

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0.05T

50µm
0T-0.05T

0100
Distance between pump light and probe light, ∆x(mm)ics of carrier spin in semiconductors has been
investigated by time-resolved optical measurements of the temporal development of theprecession.62,63) Kikkawa et al. have shown thatthe spin relaxation time can be extended to
100 ns in bulk n-GaAs with a doping densityof 1016 cm-3,64) and have demonstrated thatspin can be transported over macroscopic
scales of 100 µm without loss of coherence,as shown in Fig. 3.65) Also, Malajovich et al.
have shown that spin coherence can even betransported across semiconductor junctions
with different band gaps and g values, such
as GaAs/ZnSe heterojunctions.66)
It has been shown that electron spin coherence is maintained for periods of at least a
few nanoseconds in n-type ZnCdSe/ZnSequantum wells,67) in GaAs/AlGaAs (110) quantum wells, 68) and in GaN,69). These observations raise the possibility of application of confined electronic states to spin memory andnuclear spin manipulation.

3.5 Interaction between carrier
spin and nuclear spin
To design and implement a solid state
quantum computer that uses nuclear spin asa quantum bit, it is necessary to clarify the
dynamics of interaction between electron spinand nuclear spin. So far, in semiconductors
and its quantum structures, electron spin polarization have been measured to investigate
the hyperfine interaction involving theOverhauser effect and the Hanle effect,41,70,71)
and over the last few years more light has been

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similar order to that of II-VI isolators near theband edge,76) and are thus promising materials for use in integrated optical isolators.
It is also possible to increase magneto-optical activity by introducing fine particles of
MnAs (semimetal), which is ferromagnetic atroom temperature, into thin films of GaAs.77)
Magneto-optical effects can further be enhanced by incorporating one-dimensional
semiconductor photonic crystals.78) Model calculations have shown that the resulting Faraday effect is determined by the loss associated with the MnAs fine particles and the confinement of the electromagnetic field by thephotonic crystal.
The fabrication of (Cd,Mn)Te magneto-optical waveguides on GaAs substrates has also
been reported,79) and it has been confirmedthat the waveguide mode can be changed byvarying the external magnetic field.80)

B (T)Magnetic field B (T)

Fig. 3 Coherent transport of spin
packets when an electric field isapplied to n-GaAs,65) according to
the time-resolved Faraday rotation
measurements. The vertical axis
shows the magnetic field B, andthe horizontal axis shows the
distance ∆x between the pumplight and the probe light. The
measurements were made at 1.6
K. The excited electron spinsprecess about the magnetic field,
and a large rotation angle (i.e. spinpolarization) is observed when the
period of this precession matches
the period ∆ t of the excitation
pulses. At time t, the spin phaseof electrons excited by the nthpulse is given by ω(t+n∆t), whereω is the Larmor frequency. The
inset shows the electron spincomponents generated at ∆x, nperiods previously (i.e. at t-n∆t).

shed on the dynamics of the Overhauser effect from the time-resolved pump-probe measurements.72-74)

4. Optical Spin Devices
4.1 Optical isolators
Light with a wavelength of 0.98 µm is going to be used for the excitation of the latest
erbium-doped optical fiber amplifiers that support internet backbones. This wavelength region is absorbed by iron, so it is not possible touse conventional garnet single-crystal opticalisolators. Semiconductor optical isolators basedon a II-VI DMS, (Cd,Mn)Te, which has low absorption and a large Faraday rotation, is being
developed. Since a II-VI DMS is paramagnetic
at room temperature, a magnetic field is needed
to obtain Faraday rotation. A Verdet’s constantof 0.05 deg/Oe·cm is currently achievable at
0.98 µm using (Cd,Mn,Hg)(Te,Se). In this case,an insertion loss of 0.8 dB can be achieved at
the module level. This is the first commerciallyavailable semiconductor spin electronics device.75)

4.3 Ultra-fast optical switches
Tackeuchi et al. were the first to apply circularly polarized pump probe methods for themeasurement of spin relaxation in semiconductor quantum structures, and have shownthat the spin relaxation time of an AlGaAs/
GaAs quantum well is 32 ps.81) When spin polarization is generated in a quantum well by
an external light, the absorption for right andleft circularly polarized light changes while thepolarization is present. By using this effect toextract optical signals only when spin polarization is present, very fast optically controlled
gate switches can be constructed. In particular, in quantum wells containing different V
atoms between barrier and well, such as
InGaAs/InP quantum wells, the spin relaxation
time is no more than a few ps at room temperature. It has also been shown that ultrafast optical switching can be achieved entirely
optically by optically extracting the difference
between left and right circularly polarized lightcomponents.

4.4 Photoinduced magnetism
4.2 Integrated optical isolators
When the bandwidths of network reachGb/s at the end users, it becomes necessary
to integrate lasers and isolators using the same
semiconductors to realize high-performancelow-cost devices. Magnetic semiconductors
based on III-V compounds, such as (Ga,Mn)As,(In,Mn)As and their mixed crystals, can be integrated together with GaAs- and InP-basedlasers, and have a large Verdet’s constant of a

Koshihara et al. showed that ferromagnetism can be induced by photogenerated carriers at low temperatures in heterostructures
made of magnetic semiconductor (In,Mn)Asand non-magnetic semiconductor GaSb. 83)
This effect is illustrated in Fig. 4(a) and (b).
Since the (In,Mn)As layer is only 12 nm thick,the incident light is absorbed by the GaSb
layer, where it produces electron-hole pairs
which are split by the internal electric field,

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causing the holes to accumulate in the(In,Mn)As layer at the surface. Since (In,Mn)As
exhibits hole-induced ferromagnetism, theholes generated by the incident light cause atransition to a ferromagnetic state by presetting the hole density in the (In,Mn)As film sothat it is already on the point of becoming
ferromagnetic. Although this mode of operation is only achieved at low temperature, it
opens up a range of new possibilities and is
attracting considerable interest as a result.
On a related theme, Haneda et al. havegrown GaAs with Fe particles, and have observed optically induced magnetization atroom temperature.84) Akinaga et al. have discovered optically-induced magnetoresistanceeffects at room temperature in a structure
where granular MnSb crystals are grown on a
GaAs (111)B substrate and capped with
GaAs.85)
It should be noted that if a magnetic semiconductor laser can be produced, then it will
be possible to vary its wavelength by an external magnetic field. For example, the wavelength of an optically excited (Cd,Mn)Te multiple quantum well laser has been shown toshift to lower energies at a rate of 3.4 meV/T.86) If a large effective g value is achieved atroom temperature in an active layer of semiconductor lasers, then such devices should finda number of applications starting in the fieldof spectroscopy.

magnetic semiconductor tri-layer structure, aspin-dependent scattering effect similar to thatof metallic magnetic structures is observed
whereby the resistance increases when thetwo ferromagnetic layers have antiparallel
magnetization. Figure 5 shows an example ofthe spin-dependent scattering measured in a
(Ga,Mn)As/(Al,Ga)As/(Ga,Mn)As tri-layer
structure.90) Due to differences in the coercive
force, the region where the magnetizationsof the ferromagnetic layers are antiparallel
appears as a stepped part in the hysteresis of
the magnetization curve obtained by Hall measurements. In this antiparallel region, the sheet
resistance is found to increase due to spindependent scattering. Although the magnetoresistance ratio is small, it is important tonote that the spin does not relax in the valence band where a large spin orbit interaction is present. Minor loops measured in thesame tri-layer structure showed the presenceof ferromagnetic interlayer coupling. Resultsof polarized neutron reflectometry on(Ga,Mn)As/GaAs superlattices also showed thepresence of the ferromagnetic interlayer coupling between (Ga,Mn)As layers, which may
be mediated by holes90).

5. Electron Spin Devices
The large magnetoresistance effect arising from spin-dependent scattering in multilayered films of magnetic metals87) is now
being used in the magnetic read heads ofhard disks. A large tunnel magnetoresistanceis also obtained in tunneling junctions where
an insulating film is inserted between magnetic materials,88,89) which is likely to find applications in magnetic sensors, magnetic
memories and recording elements. If magnetic transport characteristics such as these
are implemented using ferromagnetic/nonmagnetic semiconductor heterostructures,
then it should be possible to integrate theminto existing semiconductor circuitry. Thissection discusses the magnetic transportcharacteristics of magnetic/non-magnetic
semiconductor heterostructures.

5.1 Spin-dependent scattering andinterlayer coupling
In a ferromagnetic/non-magnetic/ferro-

0.3Hall resistance, RHall (kΩ)

Hall resistivity, ρHall(µΩm)

(a)

4
0
-4
-8
1

(b)

0
-1
-2
-0.30

2.585
XAI=0.14
B⊥plane

0.2

T=30K
0.1

0.0

2.580

-0.1

-0.2

-0.3
-0.010-0.005

0.000

0.005

Sheet resistance, Rsheet (kΩ)

Magnetization (10 -2T)

8

2.575
0.010

Magnetic field B (T)
-0.15
0.0
0.15
Magnetic field H (T)

0.30

Fig. 4 (a) Magnetization curves and (b) Hall resistivity of a 12 nm thick (In0.94Mn0.06)As layer
on a GaSb buffer layer at 5 K, before and after exposure to white light. The white andblack circles in (a) correspond to the results obtained before and after exposure, respectively.
The layer is paramagnetic before exposure, but after exposure the magnetization increases
and it exhibits distinct hysteresis, indicating that it has become ferromagnetic (the theoreticalcurve is shown by the solid line). (b) Similar behavior appears in the Hall resistance-after
exposure (solid line) it exhibits hysteresis that was not apparent before exposure (dashedline).83)

Fig. 5 The magnetic field dependence of Hall resistance (ο) and sheet resistance (♦) in a
(Ga,Mn)As/(Al,Ga)As/(Ga,Mn)As tri-layer structure. The magnetic field is applied
perpendicular to the sample surface. By employing a Al composition 0.14 in the (Al,Ga)As,and by using a buffer layer of (In,Ga)As, the easy axis of magnetization is oriented
perpendicular to the sample surface. In this way, by applying a magnetic field perpendicular
to the sample surface, the anomalous Hall effect can be used to measure the parallel/
antiparallel state of the magnetization. The region where the stepped part can be seen in
the Hall resistance corresponds to the state where the magnetization is antiparallel, and in
this region the sheet resistance is found to increase due to spin-dependent scattering.90)

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5.2 Tunneling magnetoresistanceeffects and tunneling effects
Tunneling effects have also been observedin (Ga,Mn)As/AlAs/(Ga,Mn)As tri-layer structures.91-93) Figure 6 shows a recently reported
example.93) In the stepped part where the magnetization is antiparallel, the tunneling resistance increases. When the AlAs film that constitutes the tunneling barrier is four-monolayer
thick, tunnel magnetoresistance ratio over
70% has been obtained. This suggests thatthe spin polarization in (Ga,Mn)As has a highvalue of at least 50%.
Splitting of the emitter’s valence band due
to spontaneous magnetization has also been
observed by using a resonant tunneling diodewith a (Ga,Mn)As emitter.94,95)

5.3 Magnetoresistance effects inmagnetic material/semiconductor
hybrid structures
Unique magnetoresistance effects are expected from systems where fine particles ofmagnetic material are embedded in or formedon the surface of a semiconductor. In the caseof (Ga,Mn)As, fine particles of MnAs separate
out into the (Ga,Mn)As matrix when it is subjected to heat treatment.96) However, the presence of MnAs has little effect on the roomtemperature transport characteristics, possibly because of a Schottky barrier formation
around the MnAs particles. On the other hand,
Akinaga et al. investigated the magnetic fielddependence of current-voltage characteristics
in molecular beam epitaxially grown MnSb
clusters on the sulfur treated GaAs substrate,
and observed a very large magnetoresistance
up to a magnetoresistance ratio of 10,000%
at room temperature.97) The resistance observed here is positive; it increases as a magnetic field is applied. The mechanism behind
this expression of magnetoresistance has yet
to be clarified.

5.4 Controlling ferromagnetism
with an electric field
Recently, we have used field-effect transistor structures to vary the hole concentrations in magnetic semiconductor layers, andhave successfully turned the carrier-induced
ferromagnetism on and off by varying the electric field without changing the temperature.98)
Using a channel consisting of a 5 nm thickness of (In0.97Mn0.03)As, we varied the hole concentration by using a polyimide insulating
gate, and observed the channel magnetization via the anomalous Hall effect.
Figure 7 shows the magnetic field dependence of Hall resistance at 22.5 K in the vicinity of the ferromagnetic transition temperature Tc. When a gate voltage of VG = +125 V isapplied, the magnetization decreases and thehysteresis disappears. On the other hand,when a voltage of VG = -125 V is applied, thehysteresis becomes more clearly apparent andthe magnetization increases. This demon-

2

Resistance [Ω-cm ]8K
TMR ratio=72%

0.024minor loop

0.022
0.02
0.018
0.016
0.014
-0.02

-0.01

0

0.01

0.02

Magnetic field applied in the [100] direction (T)strates that the device can switch ferromagnetic phase on and off by the applied electricfield that changes carrier concentration in thechannel region. As the figure shows, a swingof VG = -+125 V in the applied voltage modulates the number of holes by ± several percent, and Tc is modulated by approximately±1 K.

6. Quantum Spin Devices
One of the goals of semiconductor spinelectronics is to implement quantum information processing based on semiconductor devices using the long spin coherence times ofelectron spins and nuclear spins. The use of
semiconductors has various benefits for implementing quantum computers-not only are
they solid state materials and suitable for largescale integration, but they also allow dimensional freedom to be controlled by quantum
confinement, and allow various characteristics to be controlled by external fields such as
light and electric or magnetic fields. This section introduces proposals of solid-state quantum computers using spin in semiconductors.

6.1 Quantum computing using
spins in quantum dots
A number of proposals have been madefor quantum computers using a single-electron spin state in quantum dots as quantum
bits, since it offers a two-level system close to
the ideal case and it has relatively long coherence time.99-101) To distinguish one quantum
bit to be operated on from other quantum
bits, the resonant frequency is shifted by, forexample, applying a local magnetic field andthis frequency is used for operation on thequantum bit. In a quantum computer proposed by Loss et al., instead of subjecting thesystem to external electromagnetic waves, thetemporal development of local exchange interactions between the target quantum dot
and other neighboring quantum dots or ferromagnetic materials is introduced by a gate
allowing the spin to be manipulated.99) The
use of solid-state (semiconductor) technology
is thus advantageous in the way that it allows
spin to be controlled electrically.

6.2 Quantum computing using
nuclear spin
Fig. 6 Tunnel magnetoresistance effect in a (Ga,Mn)As/AlAs/(Ga,Mn)As tri-layer structure, measured at a temperature of
8 K. The solid and dotted arrows indicate positive and negative magnetic field sweeping directions, respectively. The(Ga,Mn)As layers are 50 nm thick, and the Mn composition in these layers are 0.04 and 0.033. The AlAs layer is 1.6 nm
thick Since the easy axis of magnetization lies within the plane of the sample, a magnetic field is applied parallel to thesample surface. When a magnetic field is applied along the [100] direction, a tunneling magnetoresistance effect of over
70% is observed, and when a magnetic field is applied along the [110] direction, this effect is approximately 30%.93)

10

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For silicon, the majority isotope 28Si has
no nuclear spin. Therefore, any intentionally
doped nuclear spin stands out and has a longrelaxation time. Kane has proposed a Si-based

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Fig. 7 Gate voltage dependenceof Hall resistance in an(In,Mn)As FET structure close to
the ferromagnetic transition
temperature. When a gate
voltage of VG=-125 V is applied,hysteresis is clearly observed. On
the other hand, when VG=+125
V, the hysteresis disappears. This
demonstrates that the appliedfield is able to switch the device
between ferromagnetic andparamagnetic states.98)

50
T = 22.5 K

R Hall (Ω)

2506.4 Detecting spin
To obtain the final results of a quantum
computation, one needs to read out a single
nuclear spin or an electron spin state. Various
methods have been proposed for doing this,such as spin filters using tunneling barriers offerromagnetic material,107) and a method thatinvolves using a single electron transistor (SET)
to read out the spatial distribution of an electron wave function depending on the spinstate.108) The development of such device technologies is an important area for future workin the realization of quantum computers.

0V+125 V-125 V
0V

-25
-50
-1.0

which is long enough to allow computations
to be performed provided that suitable error
correction is applied.

-0.5

0

0.5

1.0

B (mT)

7. Conclusion
quantum computer that uses a silicon MOS
structure with a doped 31P (nuclear spin 1/2),
as shown in Fig. 8.102) The nuclear spin of 31P
acts as a quantum bit (qubit). It also acts as adonor that binds an electron. To manipulate
the target’s nuclear spin, a voltage is applied
to a gate electrode located above the 31P or
between the neighboring 31 P. This way thehyperfine interactions, which are determinedby the overlap of electron wave functions andnuclear spin, are controlled and shift in nuclear
magnetic resonance frequency is obtained.102)
To implement a quantum computer thatuses nuclear spin, isotopic control of the host
semiconductor crystal is essential. So far useful experimental results such as greatly improved thermal conductivity have been obtained by controlling the isotopes.103,104) Recently, Ito et al. succeeded in refining silicon
to produce single crystals consisting of
99.924% 28Si, which has much greater isotopic purity than natural silicon (where the abundance ratio of 28Si is 92.2%).105)

ing.

Semiconductor spin electronics (semiconductor spintronics) is the subject of increasingly intense study in two directions: semiconductor magneto-electronics, which is expected
to produce practical results in the near future,and semiconductor quantum spin electronics,
which is set to become an important research
topic for the 21st century. In this paper wehave reviewed the significant progresses madein this field over the last 10 years, and covered possible applications of this emerging
technology. We now have a better understanding of and control over spin related phe-This method is suitable for high-speed operation because the manipulation of quantum
bits can be performed at the electron spin resonant frequency. Another advantage is that,since it uses Ge, the effective mass of electrons is smaller and the device dimensions canbe increased to a size capable of being fabricated by existing micro-processing techniques(about 200 nm). Although the spin-orbit interaction in Ge is larger compared to that in
Si, the electron spin coherence time is expected to be similar to that of Si (about 1 ms),

BAC (≅10 -3 Tesla)T≅100 mKB (≅2 Tesla)
J-Gates
A-Gates

6.3 Electron spin quantum computers
Yablonovitch et al. have proposed an electron spin resonance (ESR) transistor based on
Si/SiGe.106) In their model, a qubit is an electron spin state bound to a donor impurity in aquantum well consisting of two SiGe layerswith different compositions. To manipulate thespin, a gate voltage is applied so that the electron wavefunction is localized to one of the
SiGe layers. Since Si and Ge have different g
values, the resonant frequency of the localized electron spin can be controlled by shifting the position of the wavefunction by gat-

Barrier
Si
31

P+

e-

e-

31

P+

Substrate

Fig. 8 A schematic illustration of a quantum computer proposed by Kane, in which nuclear spins 31P+ embedded in a
isotope-free 28Si is used as a quantum bit.102) The overlap between the electrons and the 31P+ is controlled by the A gates,
thereby varying the resonant frequency of the nuclear spin via hyperfine interaction. The J gates control the overlap of
the electron wavefunctions between two neighboring 31P+, allowing the manipulation of two quantum bits.

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nomena in structures consisting of magnetic
and semiconductor materials, such as III-V ferromagnetic semiconductors, and in non-magnetic semiconductor quantum/nanostructures.
We are, however, at the very early stage of alearning curve on the use of spin degree of
freedom in semiconductors. The Japanese research community has been supported by aprogram “Spin Controlled Semiconductor
Nanostructures” (1997-1999)* from the Ministry of Education and has contributed in a

number of aspects of semiconductor spin electronics, especially in the area of materials science and physics. This research field is currently relying heavily on a US DARPA program
called SPINS (Spins in Semiconductors, Program Manager Stuart Wolf) started in the year
2000, which supports a broad spectrum of
research mainly in the United States. In orderto develop semiconductor spintronics further,in particular quantum spin electronics involving the manipulation of coherent spin states,

which may take decades to develop, a varietyof comprehensive and dynamic research anddevelopment programs are needed. They haveto involve specialists from a broad spectrumof fields ranging from materials, device andcircuit technology to system design and computer science. We sincerely hope that this review article is of some use in raising interestsof graduate students and researchers as wellas establishing links with researchers in otherfields, as it is vitally important for the further

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development of the field as electronics.
* See http://www.ohno.riec.tohoku.ac.jp/ fora summary of the report of the program.

on Priority Area (Spin-Controlled Semiconductor Nanostructures) and Basic Research (A).

Note added in proof
Acknowledgments
The research at Tohoku University was
supported by the “Research for the Future”
program of the Japan Society for the Promotion of Science, and by Grant-in-Aids from
the Ministry of Education, Culture, Sports,
Science and Technology, Scientific Research

Since this article, which is a translation ofan article written in Japanese, was completed
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